There is a requirement in industry for the measurement of conditions such as strain or temperature and other conditions at all points over long distances. Typical uses are for monitoring oil and gas wells, long cables and pipelines. The measurements can be displayed or analysed and used to infer the condition of the structures. Distributed temperature sensors (DTS) often use Raman or Brillouin components of scattered light in optical fibers as the means to determine the temperature. Here, light from an optical source is launched into a fiber and the small amount of light that is scattered back towards the source is analysed. By using pulsed light and measuring the returning signal as a function of time, the backscattered light can be correlated to distance along the fiber. This backscattered light contains a component which is elastically scattered (Rayleigh light) and components that are up- and down-shifted in frequency from the source light (Raman and Brillouin anti-Stokes and Stokes light respectively, also known as inelastic scattered light). The powers of the returning Raman components are temperature dependent and so analysis of these components yields the temperature. The powers and frequency of the returning Brillouin components are strain and temperature dependent and so analysis of both components can yield temperature and strain independently. Such systems have been known for many years.
A typical optical fiber is composed of a core within a layer of cladding and thereafter one or more buffer layers. The core provides a pathway for light. The cladding confines light to the core. The buffer layer provides mechanical and environmental protection for both core and cladding. A typical single-mode fiber (SMF) is composed of precision extruded glass having a cladding with a diameter of 125 μm+−2 μm and a core with a diameter of 8 μm+−1 μm at a centre of the cladding. The buffer layer is typically composed of a flexible polymer applied onto the outer surface of a cladding. Most commercial fibers are manufactured with a buffer layer of a polymer coating. With special polymer materials such as polyimide, these types of fiber can offer good performance up to 300° C. in normal atmosphere. However, above 300° C. and in high water or high content hydrogen environment, the performance of the optical fiber is significantly degraded due to deterioration of the coating and or ingress of hydrogen. It is known to use metallic coatings on fibers for higher operating temperatures and more resistance to hydrogen ingress. In this case the fiber is pulled through a pot of molten metal that has a melting point less than the fiber. A thick metallic coating is formed around the fiber as the metal solidifies. However, because of the difference in expansion coefficient of metal and fiber material, the metal coating exerts addition strain on the fiber that commonly results into higher optical losses. Another problem is the ability to produce long continuous length of the fiber due to the higher probability of weak points being induced in the fiber by the metallic coating, and limitations of the capacity of most common coating devices.
It is also known that fiber optic cables can deteriorate in harsh environments such as those encountered in down-hole fiber optic sensing applications. As discussed in U.S. Pat. No. 6,404,961, down-hole environmental conditions can include temperatures in excess of 130° C., hydrostatic pressures in excess of 1000 bar, vibration, corrosive chemistry and the presence of high partial pressures of hydrogen. The deleterious effects of hydrogen on the optical performance of optical fiber, particularly in sub-sea installations for the telecommunications industry, have long been documented. To protect optical fibers from the effects of hydrogen, hermetic coatings and barriers, such as carbon coatings and the like have been used to minimize the effects of hydrogen. However, such submerged environments are cold. At the elevated temperatures experienced in a harsh down-hole environment, such coatings lose their resistance to permeability by hydrogen. Additionally, at such high temperatures, the effects of hydrogen on an optical fiber may be accelerated and enhanced.
U.S. Pat. No. 6,404,961 suggests using a core including an inner stainless steel tube having one or more optical fibers contained therein, and a surrounding protective layer includes an outer stainless steel tube and a layer of buffer material such as teflon positioned between the outer tube and the inner tube, the buffer material maintaining the inner tube generally centrally located within the outer tube and providing a mechanical link between the inner tube and the outer tube to prevent relative movement therebetween. The inner steel tube may be coated with a low hydrogen permeability material such as carbon, to minimize the entrance of hydrogen. The carbon can be coated with a protective layer of polymer to protect against damage such as scratching of the carbon.
Another proposal is shown in WO2004066000. This explains that since the optical fiber itself is relatively delicate, special care must be taken to protect it as it is being placed in the well bore and during normal operation of the well. One known method is to install a small hollow metal tube, sometimes referred to as a capillary tube or instrumentation tube, having an outside diameter of approximately ¼ inch, down the well as it is being completed. Such tubes are also typically installed in well bores for other purposes, such as chemical injection. The fiber optic cable is typically comprised of a glass or plastic fiber core, one or more buffer layers, and a protective sheath. The optical fiber is typically a single optical fiber strand, coated with a thin layer of a protective material, typically composed of a heat polymerized organic resin. This may be impregnated with reinforcing fibers for installation in well bores where the operating temperatures may reach 250° C. The fiber optic cable may need to be installed at lengths of up to 40,000 feet. State-of-the-art apparatus for installing such fiber optic cable typically include means for pulling the cable from a cable reel, propelling the cable by means of tractor gears, or a capstan, and in some cases, impelling the cable through the duct by means of fluid drag. Following completion of the well, an optical sensing fiber is installed inside the instrumentation or capillary tubing by pumping a fluid down the tubing and using the velocity of the fluid to drag the fiber down the tubing.
One advantage of this approach is the ability to replace a failed optical fiber, by pumping it out and re-pumping in a new one without interrupting the normal operation of the well. However, the fluids used to pump the fiber down the instrumentation or capillary tube may be harmful to the optical fiber and lead to failure of the optical fiber over time, especially at the elevated temperatures typically seen within a well bore. Although pumping the fiber out of the well, and deploying a new fiber is possible, as described above, the procedure is time consuming and expensive even though the well continues to operate during the removal and re-deployment of the fiber. Also, there is the risk that the replacement operation is not successful.
Further, it is well known that any moisture (water) present in the instrumentation or capillary tube will also seriously attack the integrity of the optical fiber at elevated temperatures. In addition, hydrogen gas, normally found in many oil and gas wells, tends to seep into the instrumentation or capillary tubing over time. The hydrogen gas is absorbed by the optical fiber, causing the fiber to darken. The end result of the above described processes is that the optical fiber fails regularly when subjected to high temperatures within the well bore, sometimes in a matter of days, and has to be replaced.
WO2004066000 proposes a flexible protective barrier around the core and cladding of the optical fiber, the protective barrier being sufficiently flexible to allow storage of the fiber assembly in a spooled condition, and having an outer diameter sized to easily fit within an instrumentation or capillary tubing through which the fiber assembly is pumped down into a well bore. The protective barrier can be thin tubing formed from nickel or stainless steels, or other materials that prevent the transmission of deteriorating substances into the fiber. The protective tubing may include a hydrogen scavenging material coating on the inside or outer side of the tubing. The tubing that encases the optical fiber protects the fiber during deployment and during actual operation, and is significantly more robust than bare fiber, having a breaking strength more than 10 times that of a typical optical fiber. Moreover, the fiber inside the tubing is not exposed to the injection fluids used to pump it down the instrumentation tubing, minimizing exposure to injection fluids as a source of fiber degradation. Further, since the tubing encasing the fiber can be hermetically sealed, moisture can be kept away from the fiber, thus eliminating another major source of optical fiber failure. Alternatively, the fiber tubing can be filled with an inert gas such as nitrogen.
One particularly harsh environment is bore holes used for high temperature steam recovery, namely Steam Assisted Gravity Drainage (SAGD) and Cyclic Steam Stimulation (CSS) as is becoming widely used for oil recovery in Canada. In order to fully understand and optimise recovery, real-time monitoring has become an integral part of the SAGD and CSS processes. The monitoring measures temperature and pressure using fiber optic Distributed Temperature Sensors (DTS) which have the ability to take measurements every 1 m with a resolution of better than 0.01° C. At present the method of installation for the fiber optics is to install a ¼″ control line inside the well and to “pump” the fibers into the control lines. The outer protective coating of the fibers has been a combination of polyimide and carbon coated fibers which are rated by the manufacturers to 300° C. under lab conditions. However, in practice the fibers often fail at much lower temperatures (below 200° C.). Fiber darkening, leading to the total loss of signal, has been seen to occur much sooner than expected, over periods ranging from hours to months.
For accurate distributed sensing it is important to be able to account for variations of the optical fiber properties and correct for any changes that can result in a measurement error. For distributed temperature sensing, if no appropriate calibration is made, the variation of differential loss between the Stokes and anti-Stokes results in a temperature error in the computed temperature along the fiber. Also, if the loss is very high, as has been observed in some wells, it is not possible for a usable amount of light to pass down the fiber, making it impossible to take any measurement.